The present invention relates generally to the art of manufacturing lead monoxide (PbO) with widely varying free metallic lead content. More specifically, the invention relates to a horizontal cylindrical reactor and its method of operation wherein a series of horizontally disposed rotating blades are utilized to effect the oxidation reaction and to simultaneously grind and classify the lead oxide particles for transfer through and out of the reactor in an induced air draft.
Lead monoxide (also variously referred to as lead oxide or litharge) is an extremely important compound in a variety of industrial manufacturing processes. For example, the largest single use of lead oxide is in the manufacture of lead-acid storage batteries. Large volumes of lead oxide or litharge are also used as fluxing agents in the manufacture of glass. These and various other manufacturing processes require lead oxide with free metallic lead contents ranging from relatively high (18-35%) for the manufacture of batteries to very low (less than 1%) for the manufacture of glass. A variety of old and well-known processes and apparatus have been in use for many years to produce lead oxide for its various applications.
One old and well-known process utilizes the so-called Barton pot as disclosed in U.S. Pat. No. 633,533. In the Barton process, a large heated pot is fed with a stream of molten lead which is maintained at a shallow depth in the bottom. A rapidly rotating blade in the bottom of the pot continuously agitates the molten lead which is oxidized in the presence of a stream of air and water or steam. The oxidized lead particles are drawn from the pot by the air stream while the heavier lead droplets fall by gravity to the bottom of the pot for further agitation and oxidation. The process is controlled by adjusting the rate of feed of the molten lead, the air flow through the pot and the speed of the blades. The Barton process operates at high temperatures, substantially above the melting point of lead which is 327.degree. C. (620.degree. F.). Though quite versatile, the Barton process has several inherent disadvantages. When used for producing high free lead litharges for battery manufacturing, the leady oxides produced are often too coarse for use in the formulation of battery active material paste and must, therefore, be subjected to hammer mill processing subsequent to their initial production in a Barton pot. In addition, the Barton process is difficult to control when used to produce low free lead litharge and cannot be used to consistently produce lead oxide with a free lead content of less than 1%. Finally, the operation of a Barton pot results in a build-up of lead on the blades and walls, requiring periodic shut down and manual cleaning of the component surfaces. A more recent version of this apparatus is disclosed in U.S. Pat. No. 3,322,496. However, unlike the earlier Barton pot, in this device no reservoir of molten lead is maintained in the bottom.
Another widely used process for the manufacture of lead oxide employs a horizontal rotating attrition-type mill. One of the most common types is the Shimadzu mill named after the inventor in U.S. Pat. No. 1,896,020. In the Shimadzu mill, lead oxide is produced by tumbling pieces of metallic lead in a rotating drum in the presence of a stream of air. The heat generated by oxidation and the frictional heat of tumbling provides a self-sustaining reaction. The smaller and lighter oxide particles which are produced may be withdrawn from the mill by various means, one of which may simply be an induced draft of air drawing the lead oxide particles through an outlet in one end. The Shimadzu and related processes are considered low temperature, operating below the melting point of lead. Although relatively high production rates can be attained from a Shimadzu mill, the process is highly energy inefficient because it requires the tumbling of a large mass of metallic lead within the drum. Further, because it is a relatively low temperature process, the Shimadzu mill is not suitable for the manufacture of low free lead litharge.
U.S. Pat. No. 1,204,537 discloses another type of reactor and method for its operation to produce lead oxide. In this apparatus, an inclined frustoconical reactor chamber has disposed therein a rotatable axial shaft carrying a series of composite blades adapted to rotate in close proximity to the inner reactor surfaces. The lower end of each blade has a paddle-like structure which rotates through a lower reservoir of molten lead and lifts and disperses lead droplets into the upper reactor chamber, in a manner somewhat analagous to the operation of the Barton pot described above. The upper portions of the blades act as mixing devices and simultaneously induce a flow of preheated air through the reaction chamber to oxidize the lead particles and transport the same to an upper reactor outlet. Partly oxidized lead and lead particles are returned by gravity to the molten lead reservoir. The operation of this device requires a high energy input and the inherent variation in the edge speed of the tapered mixing blades along the length of the frustoconical reactor results in a nonuniform and ineffecient reaction zone. Correspondingly, any centrifugal classification of the lead oxide is likewise nonuniform. The apparatus disclosed has no known industrial application, either past or present.
German Pat. No. 266,348 discloses a reactor for the production of litharge which is similar to the Barton reactor described above. In this apparatus, however, the mixing blades or paddles are disposed to rotate about a horizontal axis with their edges or tips dipping into the surface of the molten lead in the pot. The operation of this device is otherwise similar to the Barton reactor and is subject to the same operational deficiencies noted above. In particular, the reaction zone is small in comparison to the overall size of the apparatus and classification of the litharge produced by the oxidation reaction is primarily by gravity.